The present invention generally relates to a method for measuring filler dispersion in a polymer sample, and, more particularly, relates to a method for quantifying filler dispersion in a polymer sample through the manipulation of data obtained from a reflected light measurement method for analyzing filler dispersion.
The physical properties of polymer products containing reinforcing fillers are generally dependent upon not only the nature of the polymer itself, but also the quality and uniformity of the dispersion of the fillers within the polymer matrix. Typically, the polymers and fillers and any other selected ingredients are first mixed together in a xe2x80x9cmasterbatchxe2x80x9d to produce a uniformly blended compound, in preparation for a second or final pass, in which accelerators and curing agents are added so that the compounded polymer may be vulcanized as a final product. Generally, the individual filler particles exhibit a natural cohesiveness for each other, and it is the objective of processing to overcome this cohesiveness and to disperse the fillers into the polymer to as near the individual aggregate size as possible. Depending upon the application, the dispersion of the filler may be considered to be adequate when a given percentage of the agglomerates (clusters of filler aggregates) are of a size or diameter below a predetermined magnitude. For example, when dispersing carbon black fillers into rubber, in many applications the dispersion will be considered to be adequate when 95% of the agglomerates are below 10 microns in diameter.
Thus, dispersion is concerned with causing these agglomerates to separate and become uniformly scattered throughout the polymeric matrix. The degree of dispersion is the fineness of subdivision of the dispersed particles. Various methods have been developed to quantify or qualify the level of filler dispersion within compounded rubber, however, these methods are currently either too burdensome and time consuming or do not provide the degree of quantification or qualification of filler dispersion that is desired.
In the known automatic microscope electronic data accumulator (AMEDA) process filled polymer specimens are hardened overnight with molten sulfur, and are polished with different grinding agents to produce a mirror-like surface. Light is reflected off the specimen surface, and the filler agglomerates, typically carbon black, are detected by their higher light reflectance as compared to reflectance off of the polymer matrix. By moving the specimen at a fixed rate, an X-Y scan of the surface is provided, and optical density differences between the filler agglomerates and the polymer matrix are used to calculate percent carbon black dispersion. However, as mentioned, the specimens must be hardened overnight, and, additionally, the different grinding/polishing steps are unduly labor intensive.
Filler dispersion has also been quantified by measuring with a light microscope the percentage area covered by black agglomerates in microtome sections of the compounded polymer. Indeed, this method is described under ASTM D2663, test Method B, entitled xe2x80x9cagglomerate count,xe2x80x9d and, thus, serves as a standard test method for measuring filler dispersion, particularly carbon black dispersion, in polymer. In this method, the compounded rubber is microtomed into sections that are sufficiently thin to permit observation of the carbon agglomerates by transmitted light. The total cross-sectional area of all agglomerates 5 microns or larger is counted, and from the known content of carbon black in the stock, the percentage of carbon black below the 5 microns size is calculated and expressed as the percentage of carbon black dispersed. This process, however, requires freezing the compounded rubber and providing a microtome section, and, thus, is both time consuming and labor intensive.
ASTM D2663 also provides a xe2x80x9cvisual inspectionxe2x80x9d test method (test method A) and a xe2x80x9cmicroroughness measurementxe2x80x9d method (test method C). The visual inspection method generally involves tearing or cutting a polymer sample to expose a fresh surface for examination by the eye, preferably aided by a hand lens or low-power microscope. The level of dispersion is simply compared against a series of photographic standards, and, thus, provides merely a qualitative analysis of filler dispersion. The microroughness measurement method involves tracing a cut surface of a polymer specimen with a stylus to measure the amount of roughness caused by filler (carbon black) agglomerates. It is a quantitative test method, but is rather time consuming and labor intensive.
Of particular relevance to the method of the present invention is the reflected light measurement (RLM) method for measuring filler dispersion. Like the grinding/polishing method mentioned above, this method analyzes light reflected off a compounded polymer sample; however, in the RLM method, a polymer sample containing reinforcing fillers is cut by a blade, which, as it is advanced through the sample, urges the fillers to move out of the path of the cutting blade. The cut surface of the sample therefore exhibits depressions and bumps where polymer-covered fillers have been pushed ahead of the cutting blade, to either one side or the other of where the cut was made. The RLM method quantifies the level of filler dispersion by directing beams of light toward the cut surface, at an angle, such that the light beams reflected from either a depression or bump are reflected to a sensor, while light beams reflected from a smooth area of the cut surface are not picked up by the sensor. The sensor then produces an image of the reflectance from the cut surface, and this image is then compared to a standard set of images indicating dispersion ratings on a scale of 1 to 10, where 10 indicates very good dispersion. This method, while being relatively fast and requiring minimal labor, requires comparison to samples, and is therefore susceptible to error. Additionally, as filler dispersion might vary across different polymers and filler combinations, this method requires that a scale exist for the actual polymer and filler combination under study.
Andersson, Sunder, Persson and Nilsson expanded the comparative dispersion rating of the RLM method to give quantitative data as to the size and number of the disturbances (i.e. depressions and bumps) in the cut surface. See Andersson et al., Rubber World, March 1999, p.36. While such data provides some indication of dispersion, there still exists a need in the art for a less labor intensive and faster method for more accurately quantifying the level of dispersion of a filler in a compounded polymer.
In general, the present invention provides a method for determining a percent dispersion rating for fillers within a compounded polymer having a filler dispersed therein. A cut surface of the compounded polymer is prepared such that the fillers provide surface disturbances on the cut surface. Light is directed toward the cut surface, such that light incident on a surface disturbance is reflected to a sensor, and light not incident on a surface disturbance is not reflected to said sensor. From the light incident on the sensor, the percent area of the cut surface that is covered by the surface disturbances is computed, and, a percent dispersion rating is calculated as function of this percent area.
Notably, the fillers are not the surface disturbances themselves, but rather, the surface disturbances represent fillers covered by a thin layer of polymer. Surface disturbances below a certain size are considered to represent fully dispersed filler, such that, in calculating the percent dispersion rating for the filler as a function of the percent area of the cut surface that is covered by the surface disturbances, only those surface disturbances above a given selected threshold size are considered. In particular embodiments of the present invention, the percent dispersion rating is calculated according to the following equation:
DRL %=100xe2x88x92100 URF %/L
wherein DRL % is the filler dispersion rating; URF % is the percent area of the cut surface that is covered by surface disturbances greater that a selected threshold; and L is the volume percentage of carbon black in the compound.